QueryPerformanceCounter and overflows

Question

I\'m using QueryPerformanceCounter to do some timing in my application. However, after running it for a few days the application seems to stop functioning properly. If I simply

Answer 1

The problem you asked about most directly: if (t.QuadPart < time_.QuadPart) should instead be this: if (t.QuadPart - time_.QuadPart < 0)

The reason for that is that you want to look for wrapping in relative time, not absolute time. Relative time will wrap (1ull<<63) time units after the reference call to QPC. Absolute time might wrap (1ull<<63) time units after reboot, but it could wrap at any other time it felt like it, that's undefined.

QPC is a little bugged on some systems (older RDTSC-based QPCs on early multicore CPUs, for instance) so it may be desirable to allow small negative time deltas like so: if (t.QuadPart - time_.QuadPart < -1000000) //time wrap

An actual wrap will produce a very large negative time deltas, so that's safe. It shouldn't be necessary on modern systems, but trusting microsoft is rarely a good idea.

... However, the bigger problem there with time wrapping is in the fact that ticks_to_wait, ticks_passed, and ticks_left are all int, not LARGE_INT or long long like they should be. This makes most of that code wrap if any significant time periods are involved - and "significant" in this context is platform dependent, it can be on the order of 1 second in a few (rare these days) cases, or even less on some hypothetical future system.

Other issues:

if (time_.QuadPart != 0)

Zero is not a special value there, and should not be treated as such. My guess is that the code is conflating QPC returning a time of zero with QPCs return value being zero. The return value is not the 64 bit time passed by pointer, it's the BOOL that QPC actually returns.

Also, that loop of Sleep(0) is foolish - it appears to be tuned to behave correctly only on a particular level of contention and a particular per-thread CPU performance. If you need resolution that's a horrible idea, and if you don't need resolution then that entire function should have just been a single call to Sleep.